(WIP) FFI: add function to retrieve details on signature validation errors. #3922

Job	Run time
cmake-offline-googletest-src	4m 41s
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	5m 17s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	4m 43s
cmake-offline-googletest	4m 24s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	4m 23s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	4m 25s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	3m 58s
system-sexpp, sexpp shared libs on, rnp shared libs on	4m 39s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 31s
system-sexpp, sexpp shared libs on, rnp shared libs off	2m 59s
system-sexpp, sexpp shared libs off, rnp shared libs on	3m 9s
system-sexpp, sexpp shared libs off, rnp shared libs off	2m 57s
package-source	1m 21s
package	59s
debian-tests	48s
	53m 14s

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